homeostasis Flashcards
Explain what is meant by negative feedback in a living organism
It is a process that brings about a reversal of any change in conditions. It ensures that an optimum steady state can be maintained, as the internal environment is returned to its original set of conditions after any change. Any deviation from the norm or set point results in a response being taken which restores the norm. Once the norm is restored, the response is inhibited.
Distinguish between negative feedback and positive feedback.
Negative feedback monitors a change and initiates a response that will reverse the change. It will maintain constant levels.
Positive feedback monitors a change and initiates a process that will increase the change. It tends to be destabilising.
Define homeostasis
Homeostasis is the maintenance of the internal environment in a constant state within an organism, despite external changes
Explain why it is important to maintain a constant body temperature.
If body temperature is too low, enzyme activity is reduced, slowing the rate of metabolic reactions. If it is too high, enzymes will denature and metabolic reactions will stop. The highest rate of enzyme activity happens at their optimum temperature
Explain why target cells must have specific receptors.
Target cells must have specific receptors to which a protein hormone can bind. This enables the hormone to activate a process inside the cell. The receptor must be specific in shape so that the hormone can bind only to the correct cells that can respond to the hormone. Receptor and hormones have shapes complementary to each other.
Explain why steroid hormones can pass easily through the plasma membrane while protein hormones cannot.
Steroids hormones are lipid soluble so can diffuse through the phospholipid bilayer. Protein hormones are not lipid soluble so cannot pass through the phospholipid bilayer.
What is meant by the term ‘hormone’.
A molecule that acts as a chemical messenger, released from on part of an organism that has an effect in another part (target tissue)
Distinguish between first and second messengers and give one example of each.
Hormones are first messengers – they bind to receptors on the outside of a cell, eg. Adrenaline, Glucagon, Insulin. Second messengers are chemicals released in a cell as a result of a hormone binding to a receptor, eg. cAMP
Describe how blood glucose concentration is lowered.
Insulin lowers blood glucose concentration by binding to receptor proteins found on the plasma membrane of liver (hepatocytes), fat and muscle cells. This leads to several effects within the cells:
More glucose transport proteins are placed on the cell surface membrane (eg. GLUT4 - see next page for more information on this)
More glucose can therefore enter the cell
More glucose in the cell is converted to glycogen (glycogenesis) or fats
More glucose used in respiration
Distinguish between type 1 and type 2 diabetes mellitus.
Type 1 = autoimmune disease, beta cells not producing insulin, insulin needs to be injected
Type 2 = β cells don’t produce enough insulin or the target cells (eg hepatocytes) become less responsive to insulin due to insulin receptors on plasma membranes not working properly. It can be controlled initially by diet, losing weight (if necessary) and regular exercise. Glucose lowering medication can be taken if diet and exercise can’t control it. Eventually insulin injections may be needed.
Why do hepatocytes have specialised receptors for both insulin and glucagon?
Hepatocytes are liver cells - they contain a store of glycogen. When there is excess glucose in the blood the cells absorb glucose and make more glycogen – therefore they need to respond to insulin. However, when there is insufficient glucose in the blood the cells break down the glycogen and release glucose into the blood. Therefore they must also respond to glucagon.
What molecules are first messengers in the control of blood glucose?
Insulin, glucagon and adrenaline
Insulin is a protein. Why must insulin be injected rather than being taken orally?
Proteins are digested in the stomach and small intestine. If diabetics took insulin orally, it would be digested to amino acids before being absorbed.
Explain the meaning of the terms hyperglycemia and hypoglycaemia.
Hyperglycaemia means a blood glucose concentration that is too high (hyper = high or over, glyc refers to glucose, and anaemia refers to blood). Hypoglycaemia means a blood glucose concentration that is too low (hypo = low or under).
Most cells are specialised to perform their functions. The two hormones secreted by the pancreas are proteins. Describe and explain how the ultrastructure of the α and β cells in the islets of Langerhans is specialised to manufacture and secrete hormones.
Cells that are specialised to manufacture proteins such as insulin and glucagon will have a lot of ribosome and rough endoplasmic reticulum. This is because the ribosomes are the sites of protein synthesis. The cells will also have a lot of Golgi apparatus as substances such as hormones are prepared and packaged into vesicles in the Golgi apparatus. There will also be many secretory vesicles as these vesicles are used to transport the hormone to the cell surface (plasma) membrane for secretion by exocytosis. These cells are also likely to have many mitochondria to supply much ATP from aerobic respiration as an energy source for the active processes in the ce
Give 3 actions that health advisers think the food industry needs to do to reduce the risk of Type II diabetes.
Reduce advertising junk food
Improve nutritional value of products
Use clearer labelling on products
AVP
State which parts of a nephron can be found in a kidney medulla.
loop of Henle and collecting duct.
Describe the route of a red blood cell from renal artery to renal vein.
Renal artery à afferent arteriole à glomerulus à efferent arteriole à capillaries in cortex and medulla à venule à renal vein.
DOES NOT ENTER THE NEPHRON!
List the processes taking place in the kidney (i) Cortex; (ii) Medulla
Cortex - Bowmans capsule - ultrafiltration from the glomerular capillaries into capsule, Proximal convoluted tubule (PCT) - selective reabsorption, distal convoluted tubule (DCT) – reabsorption of water by osmosis ;
Medulla - Loop of Henle – Countercurrent mechanism – active transport of Na+ & Cl- to increase solute concentration of tissue in medulla, Collecting duct - Reabsorbtion of water by osmosis in presence of ADH – osmoregulation
What is the role of a podocyte in ultrafiltration?
Podocytes support the basement membrane. Increase surface area for absorption.
Explain why the process of selective reabsorption is necessary in the kidney
To reclaim molecules the body needs , eg. Glucose – was forced out of the blood at the Bowman’s capsule (due to its low molecular mass)
Explain the return at the PTC of much of the water filtered from the blood at the Bowmans capsule
Osmosis. The filtrate has a higher water potential than the blood from which it was filtered.
Explain how the medulla of the kidney is able to perform osmoregulation
The activity of the loop of Henle decreases the water potential of tissue fluid in the
medulla. Hence water leaves the collecting duct by osmosis when ADH is present.
State the roles of the hypothalamus and posterior pituitary gland in osmoregulation.
Receptors in the hypothalamus detect changes in the water content of the blood. When the water content falls, they stimulate the posterior pituitary to secrete ADH, which allows
water uptake from the filtrate in the kidney.
Suggest why the nephrons are convoluted.
To increase the length for greater surface area for absorption.
Why are there many capillaries around each nephron.
So that materials reabsorbed from the fluid in the tubule can re-enter the blood.
Explain why reabsorption from the nephron must be selective.
Some of the molecules in the nephron are waste and must be left in the fluid, eg. Urea, to be excreted. Other molecules are useful to the body and must be reabsorbed, eg. glucose
Explain what is meant by ultrafiltration.
Filtering on a molecular scale. Small molecules pass through the basement membrane, which acts as a filter while larger molecules are held in the blood.
Suggest what might happen if water is not reabsorbed from the nephron.
A large volume of very dilute urine would be produced and dehydration would occur.
Explain why the concentrations of glucose and amino acids are the same in the glomerular filtrate as in the blood plasma.
Because all the amino acids and glucose have been passed from blood plasma to the glomerular filtrate by ultrafiltration in the glomerulus.
Explain why the presence of protein in urine can be a sign of hypertension.
Proteins usually have a high molecular mass so are normally filtered by the basement membrane, and therefore remain in the blood. If this has been damaged by high blood pressure then proteins can enter Bowman’s capsule from the blood and pass into the urine.
Explain why it is beneficial to mammals living in arid regions to have higher salt concentrations in their medullas.
A higher salt concentration in the medulla means that a greater water potential gradient can be achieved between the urine in the collecting ducts and the medulla. This means that more water can be reabsorbed from collecting ducts and then pass into blood capillaries and the urine is made more concentrated There will be less urine produced and less water lost
What is meant by the term hairpin counter current mechanism
Hairpin refers to the tight loop that the nephron makes. Counter current because the fluid in the descending limb flows in the opposite direction to the fluid in the ascending limb. The opposite flow helps to maintain the gradient into the tissue that enables efficient water reabsorption. The longer the hairpin loop the greater the reabsorption of water will be.
Why must the collecting duct pass back through a region of low water potential.
This arrangement allows water to be reabsorbed from the collecting ducts back into the tissue fluid of the medulla. This concentrates the urine.
Suggest why beavers have short loops of Henle.
Beavers live beside or in water. Water is readily available and they do not need to conserve it so much.
The tissue fluid in the medulla has a lower water potential, so how can water pass from the tissue fluid into the blood by osmosis?
A higher salt concentration in the medulla means that a greater water potential gradient can be achieved between the urine in the collecting duct and the medulla. This means that more water can be reabsorbed from the collecting ducts and then pass into blood capillaries, and the urine is made more concentrated. There will be less urine produced and less water lost.
Suggest why kangaroo rats have long loops of Henle
They live in the arid regions/desert and water is not readily available. They must reabsorb as much water as possible from the filtrate. A longer loop of Henle allows a longer current mechanism which increases the salt concentration in the medulla, so more water can be reabsorbed
Alcohol is a diuretic and works by inhibiting the amount of ADH produced. Explain why drinking too much alcohol can cause a hangover.
Alcohol inhibits the release of ADH. Therefore the collecting ducts are not very permeable to water, as less aquaporins are placed in the membrane - therefore less water is reabsorbed. This means that more water is lost in urine and dehydration occurs. The ethanal produced from the metabolism of ethanol also contributes to the headache.
